Rotors and manufacturing methods for rotors

ABSTRACT

A rotor for driving, or being driven by, a fluid has rotor blades that follow a screw thread shape, all portions of all blades having the same screw pitch. This enables the rotor, or a molding pattern for use in making a mold, to be withdrawn from a mold part by a screw motion without damage to the mold part. This helps to make it economically viable to manufacture the rotor by a molding or casting process in which the mold parts are not destroyed to release the rotor, such as injection molding, for example with reinforced plastics. Additionally, in the case of manufacturing methods in which the mold is destroyed to release the rotor, the process of making or assembling the mold may be improved. In some molding processes, the ability to remove the rotor or the pattern by screw motion improves the economic viability of the process by reducing the number of mold parts required.

FIELD OF THE INVENTION

Aspects of the present invention relate to rotors of the type having a plurality of blades for acting on, or for being acted on by, a fluid, such as impellers in fans and compressors, and turbine rotors, in which a hub portion and a plurality of blades are formed in a single integral piece. Other aspects of the present invention relate to methods of manufacturing rotors.

BACKGROUND OF THE INVENTION

Many methods are known of making rotors, including various molding and casting methods. The method used in any particular case depends on a variety of factors, including the design of the rotor. In some cases, the rotor design and size may make molding and casting processes so impractical or expensive that it is preferred to machine the rotor from a solid block of material, or to assemble the rotor from parts.

Injection molding using a settable material such as plastics, resins, ceramics or glasses (possibly with reinforcing material), with a simple two-piece mold which is separated by movement along the axis of rotation of the rotor, may be used in some cases provided that there is no overlap in the axial direction between adjacent rotor blades. However, if adjacent blades overlap axially then a two-part mold cannot be opened by axial movement because of obstruction between the rotor blades and the mold parts at the positions where the blades overlap. At the positions where part of one blade comes in front of part of another blade, the part of the mold used to form the back face of the front blade cannot be removed backwards because it is obstructed by the blade behind it, and the part of the mold used to form the front face of the rear blade cannot be removed forwards because of obstruction from the part of the blade in front of it. If the angle of overlap in the circumferential direction is small and the axial separation of adjacent rotor blades is large, it may perhaps be possible to use a two piece mold with one mold piece defining the front face of each blade and the other mold piece defining the rear face of each blade, by arranging the mold pieces so that they are twisted slightly as they are pulled apart. However, if the angular extent of overlap is appreciable, such two-piece molding methods are not usable.

A further problem for the use of simple two-piece molds arises in rotor designs where the central hub or core of the rotor does not have a constant radius along its axial length. Provided that the hub is cylindrical, the mold pieces will slide over the hub surface without obstruction as the mold is opened axially. However, if the hub radius varies, for example it is conical or flared, the wider portions of the hub will foul the mold portions used to define the blade roots at hub positions having a smaller radius, of the mold part which is to be withdrawn rearwardly (i.e. in the axial direction which corresponds to increasing hub radius). It may be noted that rotors with flared hubs and overlapping blades, such as to prevent simple two-piece injection molding, are widely used, for example in the impellers of centrifugal flow compressors and fans, and also in radial flow turbines.

When two-piece injection molding is not possible, it is nevertheless possible to mold the rotor using a multi-piece mold, which typically will have a separate mold piece for each space between a pair of adjacent rotor blades. However, the large number of mold pieces, which need to be separately manufactured and assembled precisely, results in a considerable increase in the cost and complexity both of the process of making the mold and the process of manufacturing the rotors.

Interference between overlapping rotors can also cause problems in other casting and molding manufacturing processes as well as injection molding. For example, larger rotors are often made using sand casting. Sometimes a separate sand mold part is made for each space between a pair of rotor blades. However, it is known that in some cases the shape of the rotor blades and the degree of axial overlap is such that, as the mold parts are assembled, the last one of the inter-blade sand blocks to be inserted is obstructed by the previously-inserted blocks on either side of its intended position, such that the final inter-blade space has to be filled using several separate sand blocks each representing a separate part of the inter-blade space, each of which have to be inserted in turn and positioned correctly relative to each other. This considerably complicates the construction and assembly of the sand mold.

In general, axial overlap between adjacent rotor blades and non-cylindrical rotor hubs tend to complicate any molding or casting process for making a rotor. For relatively large rotors, either such complications have to be accepted or a different type of manufacturing technique has to be used, such as machining from solid or making the rotors as a plurality of separate parts which are fitted or fixed together. For smaller rotors, a further alternative process is known in which a flexible (e.g. silicone rubber) pattern for the rotor shape is used in the creation of an expendable mold from a settable material such as plaster of Paris. The pattern is dipped in the mold-forming material so as to create a mold shape defining both the front and rear faces of each blade in a single mold piece. Once the mold material has set, the pattern is pulled out of the mold and its flexibility allows it to deform so that it can be withdrawn without damaging the mold. Subsequently, the material for forming the rotor (typically, molten aluminum) is poured into the mold and once this is set the mold is broken in order to release the molded rotor. This process works satisfactorily in practice, but restricts the material from which a rotor may be made to aluminum and other low-melting point metals.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a rotor for driving a fluid, or for being driven by a fluid, by rotation of the rotor in use about an axis of rotation, the rotor comprising a hub having a blade-bearing surface and a plurality of rotor blades extending from the blade-bearing surface of the hub, the blade-bearing surface of the hub having a radial distance from the axis of rotation, at its intersection with a first plane perpendicular with the axis of rotation which first plane also intersects at least some of the rotor blades, which is less than the radius of the hub where the hub intersects a second plane perpendicular to the axis of rotation and spaced from the first plane, and wherein the shape of each rotor blade is such that all parts of the same rotor blade at the same radial distance from the axis of rotation are parts of a common respective screw-threaded spiral around the axis of rotation and all the screw-threaded spirals for all the rotor blades have the same screw pitch, defined as the distance travelled in the direction along the axis of rotation by the spiral in one complete revolution of the spiral about the axis of rotation. Mathematically, the shape of each rotor blade can be specified as meeting the requirement that dθ/dx is constant and the same for all values of r and the same for all blades, where r is the radial distance of a part of the blade from the axis of rotation, x is the distance of the same part of the blade in the direction along the axis of rotation from an arbitrary reference axial position, and θ is the angle of the same part of the blade about the axis of rotation from an arbitrary reference radial direction.

This aspect of the present invention has particular application to, but is not limited to, rotors wherein a blade-bearing portion of the blade-bearing surface of the hub is concave or flared, that is to say the radial distance of the surface from the axis of rotation increases with change of position along the axis of rotation in a particular direction, and the rate of change of the radial distance also increases with change of position along the axis of rotation in the same direction.

Provided that the rotor does not include further obstructions, in addition to the overlap of the rotor blades with one another and the increase in hub radius, the screw thread shape of the rotor blades enables the rotor to be withdrawn from a single mold piece which defines both faces of each rotor blade, by an unscrewing rotation of the rotor relative to the mold piece, without any requirement for the rotor to flex during withdrawal from the mold. This enables simplification of the design for a reusable mold, so that in many cases a rotor with axially overlapping blades can be manufactured using a two-piece mold. This in turn makes injection molding an economically viable manufacturing technique in situations where previously it had been too expensive owing to the large number of mold parts required. The availability of injection molding at reasonable cost adds to the range of materials that can be used as compared with the metals usable in a casting process. For example, injection molding enables lightweight resin, plastics, glass or ceramic materials to be used. A reduction in the mass of the rotor may be obtained by use of such materials in place of the metal (normally aluminum) used when the rotor is manufactured by metal casting into an expendable mold formed with a flexible pattern, provided that the necessary temperature and load-bearing requirements etc can be met. This is valuable in various contexts, such as where the rotor is used as a compressor impeller in a turbocharger or supercharger for an engine, where the mass of the impeller affects the amount of energy and time taken for it to be accelerated to the correct rotational velocity when a demand for air compression is made.

The use of screw-thread spiral rotor blades also allows benefits in other molding and casting processes. For example, in the case of sand molding a pattern having the same shape as the rotor could be used to make a single sand mold piece defining the shapes of all the rotor blades, and the pattern could then be removed from the sand mold by unscrewing in a similar way to the way in which the finished rotor is removed from the mold in the injection molding example discussed above. Additionally, even if it is desired to continue to use separate sand mold pieces for each inter-blade space, the screw thread blade shape means that the final inter-blade mold block can be inserted between the previously-positioned mold blocks by a screw spiral motion, thereby avoiding the need for multiple sand mold blocks to be made and individually positioned to fill the final inter-rotor space. In either case, there is a significant simplification when compared with existing sand molding techniques.

Accordingly, in another aspect the present invention provides a method of molding or casting a rotor having a hub and a plurality of blades in a single piece, in which adjacent rotor blades overlap axially so that a part of one rotor blade at a first axial position is at the same angle from the axis of rotation as a part of another rotor blade at a different axial position along the axis of rotation, the method comprising (i) filling a mold that defines a rotor having axially overlapping blades, each part of a blade at the same radial distance from the axis of rotation following a screw thread spiral shape with a common screw pitch for all parts of all blades, with a flowable material, allowing or causing the material to form a solid rotor, and removing the solid rotor from the mold by screw-motion rotation or by breaking of the mold. The flowable material will normally be a liquid that solidifies to form the rotor.

In one embodiment, the process involves injection molding with a plastics, resin, glass or ceramic material, which preferably contains a reinforcing component which may or may not be fibrous. For example, the rotor material may be a fiber reinforced plastic. In another embodiment, the mold is a sand mold which is destroyed to release the finished rotor. In this case, the rotor may be cast from metal.

In the case of a rotor having a flared hub, to be used in a radial flow device, it is normal for the parts of the rotor blades at one axial end of the rotor, where the hub is narrow and the fluid flow is generally axial, to twist around the hub as the blade extends axially along the hub. This is so that the plane of the blade is approximately parallel to the direction of relative movement between the fluid and the rotor taking into account both the rotation of the rotor and the axial flow speed of the fluid. The exact angle depends on factors such as the intended rotational velocity of the rotor in use and the intended volumetric fluid flow through the device. This twist of the rotor blades around the hub at the axial flow end defines the screw pitch of the screw-threaded blades, which must be constant over the whole of all of the blades. At the other axial end of the rotor, where fluid flow is substantially radial and the hub surface (having a much greater radius from the axis of rotation than at the other end of the rotor) has a greater radial component, this same screw pitch means that as the blade extends away from the hub surface, in a direction which includes a substantial axial component owing to the substantial radial component of the hub surface, the blade surface must be swept circumferentially around the rotor to some extent, creating a rake angle between the blade and the hub surface. Consequently, a screw pitch defined by the appropriate aerodynamic design for the blades at the axial flow end of the rotor typically results in a blade rake angle at the radial flow end of the rotor which is much more extreme than in conventional rotor designs. Somewhat surprisingly, it has been found in practice that it is nevertheless possible to design an aerodynamically effective blade shape for the impeller of a radial flow compressor with a screw-thread blade shape, notwithstanding the much sharper rake angle than conventional designs.

Additionally, it is known in rotors having a flared hub, to be used in a radial flow device, for the rotor blades at the radial flow end of the rotor, where the hub is wide, to twist around the axis of rotation as the blade extends radially along the hub. This is in order for the plane of the blade at this end of the rotor to be generally parallel to the direction of relative movement between the fluid and the rotor taking into account that rotation of the rotor, the radial flow of the fluid and the circumferential flow of the fluid. Consequently, with reference to the mathematical terminology used above, it is possible for θ to vary with variation in r, even at a constant x. The requirement of a screw-thread shape does not mean that dθ/dr should be constant for the same value of x, and the value of θ can be varied with changes in the radial distance r as required for the fluid dynamics of intended fluid flow through the rotor, provided that dθ/dx is kept constant and the same at all places on all blades. This possibility of varying θ with variation in r applies to all hub shapes.

An aspect of the present invention provides a rotor, for driving a fluid by rotation of the rotor about an axis of rotation or for being driven by a fluid so as to rotate about an axis of rotation, or a pattern for making a mold part for making a said rotor, the rotor or pattern comprising (i) a hub having a blade-bearing surface and (ii) a plurality of rotor blades extending from the blade-bearing surface of the hub,

the blade bearing surface of the hub having a first surface portion at a first radial distance from said axis of rotation and a second surface portion at a second radial distance, greater than said first distance, from said axis of rotation, the second surface portion being spaced from the axis of rotation in the same angular direction from said axis of rotation as the first surface portion and being spaced from the first surface portion in a direction parallel to the axis of rotation, both the first and second surface portions bearing at least a part of a rotor blade,

at least some of the rotor blades having a respective portion that is spaced from a respective portion of another rotor blade in a direction parallel to the axis of rotation but is at the same radial distance from the axis of rotation and is spaced from the axis of rotation in the same angular direction,

wherein the value of dθ/dx is constant and the same for the whole of all blades of the rotor or pattern (where x represents distance in a direction parallel to the axis of rotation, and θ represents angular direction perpendicular to the axis of rotation taking the axis of rotation as the origin) except for differences in the slopes of the blade surfaces arising solely from variation in the thickness of the rotor blades.

The blade-bearing surface of the hub may have a third surface portion at a third radial distance from the axis of rotation, the third surface portion being spaced from the axis of rotation in the same angular direction from said axis of rotation as the first and second surface portions and being midway between the first and second surface portion in a direction parallel to the axis of rotation, wherein the third radial distance is less than the average of the first and second radial distances.

The blade-bearing surface of the hub may be substantially parallel to the axis of rotation at the first surface portion and substantially perpendicular to the axis of rotation at the second surface portion.

In the rotor or pattern, the respective portions of at least one pair of rotor blades may extend circumferentially with respect to the axis of rotation over an angle of at least 5° subtended at the axis of rotation. The angle may be at least 10°, or even at least 15°.

Another aspect of the present invention provides a rotor, or a pattern for making a mold part for making a rotor, having a hub and a plurality of rotor blades extending from the hub, wherein,

for each rotor blade, all portions of the rotor blade at the same radial distance from the axis of rotation of the rotor form part of the same screw thread spiral, and the screw pitch of the screw thread spiral is the same at all radial distances of the blade and is the same for all of the rotor blades,

at least some of the rotor blades overlap axially at least partially in the sense that a part of one rotor blade at the same polar co-ordinates from the axis of rotation as a part of another rotor blade but is spaced axially therefrom, and

at least a part of the hub has a diameter which varies with distance along the axis of rotation.

Another aspect of the present invention provides apparatus for driving a gas, the apparatus comprising an impeller that is a rotor according to any of the aforementioned aspects of the invention.

Another aspect of the present invention provides a turbocharger or supercharger for an internal combustion engine, comprising a rotor according to any of the aforementioned aspects of the invention.

Another aspect of the present invention provides a mold part for use in making a rotor, the mold part having a recess for receiving molding material in use, said recess having recess portions that define both faces of each blade of the rotor, all parts of each said recess portion having a common value of dθ/dx, where x represents distance in a direction parallel to the axis of rotation, and θ represents angular direction perpendicular to the axis of rotation taking the axis of rotation as the origin, except for differences in the slopes of the surfaces of the recess portions arising solely from variation in the width of the recess portions in the direction of the thickness of the rotor blades, and at least some of said recess portions overlapping another said recess portion in the axial direction of the rotor.

Part of the surface of said recess of the mold part may define a blade-bearing surface of a hub of the rotor, such that said part of the surface of said recess has respective portions axially spaced from each other, with reference to the axis of rotation of the rotor, which portions are at different radial distances from said axis of rotation.

The part of the recess surface defining a blade-bearing surface may be convex so that the part of the recess surface is less parallel to the axis of rotation of the rotor where it is further from said axis of rotation.

Another aspect of the present invention provides a method of making a rotor comprising filling the recess of a mold part as aforementioned with a flowable material, causing or allowing the flowable material to solidify, and removing the solidified material from the mold part by moving it relative to the mold part with a screw motion.

The step of filling a mold part with a flowable material may comprise injecting the flowable material under pressure. The flowable material may be a settable plastics resin. The plastics resin may contain re-inforcing material.

Another aspect of the present invention provides a method of making the aforementioned mold part comprising immersing or embedding a rotor pattern in a mold-forming material, allowing or causing the mold-forming material to solidify, and withdrawing the pattern from the solidified material by movement with a relative screw motion to leave the recess in the solidified material. The mold-forming material may be sand or mostly sand.

Another aspect of the present invention provides a method of making a rotor comprising making a mold part by the aforementioned method, filling the recess left by the pattern with a molding material, allowing or causing the molding material to solidify, and releasing the solidified molding material by destroying the mold part.

The molding material may be molten metal, and the step of allowing or causing the molding material to solidify may comprise allowing or causing the molten metal to cool.

Another aspect of the present invention provides an inter-blade mold part for a mold for use in making a rotor by molding or casting, the inter-blade mold part having a first surface that defines a surface on one side of a blade of a rotor and a second surface that defines a surface on the other side of another blade of the rotor,

wherein for both of the first and second surfaces of the inter-blade part, all parts of the surface that define a blade surface of a rotor slope at a common screw pitch, with reference to an axis of rotation of the rotor, except for differences in the slopes of the first and second surfaces arising solely from variation in the thickness of the blades. The inter-blade mold part may be sand or mostly sand.

Another aspect of the present invention provides a sand mold-forming pattern for defining an inter-blade sand mold part as aforementioned.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention, given by way of non-limiting example, will be discussed with reference to the accompanying drawings.

FIG. 1 is a schematic front view of a rotor embodying the present invention.

FIG. 2 is a schematic side view of the rotor of FIG. 1.

FIG. 3 is a schematic front view of the hub of the rotor of FIG. 1.

FIG. 4 is a schematic side view of the hub of the rotor of FIG. 1.

FIG. 5 is a schematic front view of a known rotor provided for comparison.

FIG. 6 is a schematic side view of the rotor of FIG. 5.

FIG. 7 is a schematic front view of the hub of the rotor of FIG. 5.

FIG. 8 is a schematic side view of the hub of the rotor of FIG. 5.

FIG. 9 is a schematic plan view of a mold base part of a mold for making the rotor of FIG. 1.

FIG. 10 is a schematic side view of the mold base part of FIG. 9.

FIG. 11 is a schematic plan view of a mold lid part for use with the mold base part of FIG. 9.

FIG. 12 is a schematic side view of the mold lid part of FIG. 11

FIG. 13 is a schematic end view of an inter-blade mold part, together with views of the box parts that can be assembled into a pattern box for making it.

FIG. 14 is a schematic end view of an axial flow rotor that can be made by a manufacturing process embodying the present invention.

FIG. 15 is a schematic side view of the rotor of FIG. 14.

FIG. 16 is a schematic end view of a further axial flow rotor that can be made by a manufacturing process embodying the present invention.

FIG. 17 is a schematic side view of the rotor of FIG. 16.

FIG. 18 is a schematic top view of a turbocharger or other supercharger using the rotor of FIG. 1 as an impeller.

FIG. 19 is a schematic side view of the turbocharger or other supercharger of FIG. 18, with the shroud partially cut away.

DETAILED DESCRIPTION OF THE DRAWINGS

The disclosed embodiments are provided by way of example, and the invention is not limited thereto.

A first embodiment of the present invention is shown in FIGS. 1 and 2. FIG. 1 is a front view of a rotor suitable for use as an impeller in a radial flow (centrifugal flow) air compressor or pump, such as a supercharger or turbocharger for an internal combustion engine. FIG. 2 is a side view of the rotor of FIG. 1. FIGS. 3 and 4 are corresponding front and side views showing the hub of the rotor of FIGS. 1 and 2, without the rotor blades. By way of comparison, FIGS. 5 to 8 are views, corresponding respectively to FIGS. 1 to 4, of a known rotor suitable for the same use.

FIGS. 18 and 19 show the rotor of FIGS. 1 to 4 mounted as the impeller in a supercharger or turbocharger. The impeller is mounted in a shroud 10 (which is cut away in FIG. 19 so that the impeller is visible), which also defines an air inlet 12 and an air outlet 14. The impeller is driven by a drive means 16, which is a turbine (normally driven by exhaust gases) in a turbocharger and is some other drive means (such as an electric motor, or a coupling to take drive from the engine being supercharged) in other types of supercharger.

As can be seen from the drawings, both the rotor of the embodiment and the comparison rotor have a hub 1 having a concave flared blade-bearing hub surface 3, in which the radial distance of the surface 3 from the axis of rotation 4 of the hub increases with change in position in a direction along the axis of rotation, and the rate of increase of the radial distance of the surface 3 also increases with change in axial distance in the same direction, so that the direction normal to the blade-bearing surface 3 is closer to being parallel with the axial direction at points on the blade-bearing surface 3 which have a greater radial distance from the axis of rotation 4. Each of the rotors has sixteen blades 5, 7. Eight rotor blades 5 extend over the full axial length of the blade-bearing surface 3, and eight blades 7 extend over only part of the length of the blade-bearing surface 3, where the surface is widest. A hole 8 through the rotor, centered on the axis of rotation, allows it to be mounted on a shaft for rotation. Lines 9 in FIGS. 3, 4, 7 and 8 show the positions where respective blades 5, 7 contact the blade-bearing surface 3. These lines are sometimes known as the hub-lines of the blades or as the root-lines of the blades.

The individual blades 5, 7 of the rotors each wrap around the hub 1 to some extent, so that the angle θ of the blade from the axis of rotation 4 varies along the length of the blade. As can be seen in FIGS. 1 and 5, this has the result that each blade 5, 7 has a part which axially overlaps a part of another blade 5, 7. In this context, the term “axially overlaps” means that the respective parts of the two blades are each at the same radial distance r from the axis of rotation and both at the same angle θ with respect to the axis of rotation, and are separated merely by a distance x in a direction parallel to the axis of rotation. This axial overlapping of the blades 5, 7, together with the widening of the blade-bearing surface 3 of the hub 1 with change of distance in the axial direction, means that it is impossible for these rotors to be made by molding in a two-part mold with the mold parts being separated axially. For example, a mold part defining the blade rear surface of the front part of one of the full-length blades 5 cannot be removed forwardly, because the blade 5 is in front of it, but cannot be removed rearwardly because of obstruction by part of a part-length blade 7 and because of obstruction by part of the hub 1, which are both axially in line with the front part of the full-length blade 5. In the case of the rotor of FIGS. 5 to 8, any mold which could be removed from the rotor after manufacture, without destruction of the mold, would have to be a very complex shape with many separate pieces. In practice, the rotor of FIGS. 5 to 8 is made by casting it out of a lightweight metal such as aluminum in a plaster of Paris mold, and the mold is broken to release the rotor. In order to form the mold, a flexible pattern made from silicone rubber has to be used, so that the pattern can be removed from the mold without damaging the mold.

The rotor of FIGS. 1 to 4, which embodies the present invention, has blades 5, 7 each of which follow a screw-thread shape with a common screw pitch. That is to say, the change δx in the position of the blade in the direction parallel to the axis of rotation 4 for a given change δθ in angular position around the axis of rotation 4 without variation in the radial distance r from the axis of rotation 4 is constant and the same over the whole length of all blades 5, 7. As a result, if the rotor as a whole is moved rearwardly with a screw-type movement, both rotating and moving axially at the same time with the same screw pitch as the rotor blades 5, 7, each rotor blade will move along a path made up entirely of previous positions of that blade and the region rearwardly of previous positions of the blade-bearing surface 3. This means that if the blades 5, 7 and the blade-bearing surface 3 of the hub 1 are embedded in a single mold part, which defines both the front and the rear surfaces of each of the blades 5, 7 and defines the blade-bearing surface 3 of the hub 1, the rotor can be removed from the mold part by an unscrewing motion without any obstruction between the rotor and the mold part during removal. This enables a reusable mold to have a small number of parts, for example it may be a two-part mold. Consequently, manufacturing the rotor using a reusable mold is much simpler and cheaper with the rotor of FIGS. 1 to 4, embodying the present invention, than it is with the rotor of FIGS. 5 to 8.

FIGS. 9 and 10 show schematically a mold part 11 for molding the rotor of FIGS. 1 to 4. As shown by the broken lines in FIG. 10, the mold part 11 defines the entire shape of all of the rotor blades 5, 7 and defines the shape of the blade-bearing surface 3 of the hub 1. When the mold part 11 is seen in plan view, as in FIG. 9, only the hub lines 9 of the blades can be seen, and the remainder of the blade shapes are defined by slots disappearing into the body of the mold part 11 with the hub lines 9 being the openings of the slots in the surface of the mold part 11. Nevertheless, as mentioned above, it is possible to remove the molded rotor from the mold part 11 without damage either to the rotor or to the mold part 11 by an unscrewing movement, because this movement will cause the rotor blades 5, 7 to slide along the slots and out of the mold part 11. In the molding process, the shaft hole 8 will be formed by a core and the mold part 11 has a small recess 13 extending beyond the end of the rotor shape, in order to receive this core.

In the mold part 11, the slots which define the shape of the rotor blades 5, 7 are slightly tapered so that each rotor blade becomes thinner as it extends away from the blade-bearing surface 3. Consequently, during the unscrewing movement for removing the molded rotor from the mold part 11, each individual portion of a rotor blade 5, 7 will move into a portion of the corresponding slot in the mold part 11 which is slightly wider than the portion of the blade. This facilitates removal of the rotor from the mold part 11 by reducing the tendency of the blade surface to bind with the surface of the slot as the blade slides through the slot. This is equivalent to the known practice of providing a draft angle on a molded product, to facilitate separation from the mold.

FIGS. 11 and 12 show schematically another mold part 15 which is arranged to co-operate with the mold part 11 for molding the rotor of FIGS. 1 to 4. In effect, the mold part 11 of FIGS. 9 and 10 forms a mold base, and the mold part 13 forms a mold lid, closing the base.

The mold lid part 15 is formed integrally with the core 17 for forming the shaft hole 8 of the rotor. Additionally, the mold lid part 15 has another core 19 which fills part of the volume of the hub 1 of the rotor, in order to reduce the amount of material used to form the rotor and thereby reduce its mass. The core 19 is spaced from the core 17, and is also shaped so that when the mold is closed it is suitably spaced from the surface of the mold base part 11 that defines the blade-bearing surface 3 of the hub, to allow the hub to have sufficient thickness of material for robustness.

The core 19 has four recesses 21 extending radially inward from its circumference, which in use will define four corresponding protrusions on the rear surface of the hub 1 when the rotor is formed. These recesses 21 create an interaction between the mold lid part 15 and the molded rotor to allow the lid part 15 to drive the rotor in rotation in a manner similar to the manner in which a screwdriver drives a screw. After the rotor has been formed in the mold, it is initially removed from the mold base part 1 1 by an unscrewing motion driven by rotating the mold lid part 15 while withdrawing it from the mold base part 11. The mold lid part 15 also has holes 23 for ejector pins, for separating the molded rotor from the mold lid part 15 after it has been removed from the mold base part 11 by unscrewing.

In principle, either or both of the cores 17, 19 could be provided as separate pieces. However, it will normally be most convenient to make both cores integral with the mold lid part 15. As shown in the drawings, the shaft hole 8 in the rotor is cylindrical, and accordingly the corresponding mold core 17 is cylindrical. However, if an alternative cross-section is required, such as a square-section shaft hole or a splined shaft hole, the core 17 is given an appropriate shape. Because the ejector pins separate the molded rotor from the mold lid part 15 by axial movement rather than by rotational unscrewing, such alternative cross-sectional shapes for the shaft hole do not prevent removal of the rotor from the mold.

It should be noted that FIGS. 9 to 12 are schematic views showing the main features of the mold parts, and additional features familiar to those skilled in the art will also be present, such as holes for injection of the material from which the rotor is to be made, even though they are not shown.

As will be understood from the foregoing description, the rotor shape of FIGS. 1 to 4 is suitable for use in manufacturing a rotor by injection molding of a lightweight material such as fiber reinforced plastic, using a two-piece mold. As compared with the rotor shape of FIGS. 5 to 8, which cannot economically be made by injection molding and is therefore made by casting metal into a disposable mold, this allows a reduction in the mass of the rotor and consequently a reduction in the energy required to accelerate the rotor. This is advantageous in a variety of contexts. For example, if the rotor is used as the impeller in a turbocharger, where the limiting factor in impeller acceleration is normally the availability of energy to accelerate it at the initial moment of demand for turbocharger operation, reduction in the energy required for impeller acceleration may permit a faster response. If the rotor is used as the impeller of an electrically driven supercharger, the advantage provided by the reduced inertia of the impeller can be taken either as a faster acceleration of the impeller or as a reduction in the electrical current required to accelerate it. A further potential advantage in the reduced mass of the rotor is that if the rotor fractures during operation the mass of the rapidly moving parts of the broken rotor is less. In some applications it is necessary for safety reasons that the shroud around the rotor, or some other enclosure, is designed to be strong enough to contain the rapidly moving parts of a rotor that fractures during rotation. A reduction in the mass of those broken parts permits a corresponding reduction in the strength required for the shroud or other containing part, potentially permitting the shroud or other part to be made from a thinner material or itself be made from a lightweight plastic material instead of from metal. Thus the overall weight of the component containing the rotor can be reduced.

The feature that all portions of all blades of a rotor follow a common screw pitch is useful in other molding and casting processes, in addition to its benefits for injection molding. In any molding or casting process in which the mold is to be reused, rather than destroyed to release the rotor, the ability to remove the rotor from a mold part by an unscrewing movement will normally allow a significant reduction in the number of mold parts which are needed. Additionally, in manufacturing methods in which the mold is destroyed in order to release the rotor, this shape of rotor blade will normally allow simplification in the process of making the mold. For example, if a rotor having the shape of the embodiment of FIGS. 1 to 4 is made by pouring molten metal into a disposable plaster of Paris mold, the pattern used to form the mold can be withdrawn from the mold by an unscrewing movement, without the need for the pattern to flex. Accordingly, it ceases to be necessary to use a silicone rubber pattern and a rigid pattern can be used instead. The plaster of Paris mold made in this way will be substantially as shown in FIGS. 9 and 10. However, the recess 13 would be replaced by the core 17, which in this case would be integral with the mold part defining the shape of the blades 5, 7 and the blade-bearing surface 3 of the hub. The ability to use a solid pattern for forming the plaster of Paris mold provides a greater choice of materials from which the pattern may be made, and also avoids any potential manufacturing problems or inaccuracies that may result from the use of a flexible pattern.

Sand molding may be the preferred method of making an integral one-piece rotor if the diameter of the rotor is greater than about 30 cm. The sand mold for a rotor may be assembled from a large number of mold parts. For example, there may be a separate mold part for each inter-blade space together with a core defining the shaft hole 8. Each inter-blade mold part will define the shape of one surface of one blade and the facing surface of an adjacent blade, and also the portion of the blade-bearing surface 3 of the hub which lies between the two blades. Each inter-blade mold part will be made using a pattern, into which the sand is packed in order to form an appropriately shaped sand block. If a rotor having the shape of the embodiment of FIGS. 1 to 4 is to be made using sand molding, it becomes possible to make a single pattern for the rotor that defines the shapes of all the blades 5, 7 and the blade-bearing surface 3. The pattern will have the shape shown in FIGS. 1 and 2. This pattern can be used with a sand box to create a single sand mold piece having the shape shown in FIGS. 9 and 10, and the pattern can be removed from the sand mold by an unscrewing motion without damaging the mold. If desired, a separate core mold piece may be used to define the shaft hole 8. This greatly simplifies the process of making the sand mold for the rotor, for two reasons. First, the number of mold parts to be made is greatly reduced since there is no longer a separate mold part for each inter-blade space. Second, this method avoids the need for a step of assembling the mold by moving a large number of separate inter-blade mold parts into the correct positions with the necessary degree of accuracy.

Even if it is still desired to make the sand mold for such a rotor using separate inter-blade mold parts, an advantage is obtained at the time of assembling the parts into the finished mold. FIG. 13 shows an inter-blade mold part 25 for a rotor and the separated parts 27 a to 27 f of a pattern box for making the mold part. To make the inter-blade mold part 25, the pattern box parts are joined together to form a pattern box that is packed with sand to form the mold part. When the sand has hardened, the parts 27 a to 27 f are separated to release the inter-blade mold part 25 thus formed.

Because the angle of each blade with respect to the direction of the axis of rotation twists as the radial distance of the blade from the axis of rotation changes, and the blade also sweeps around the axis of rotation as its position in a direction parallel to the axis of rotation varies, the inter-blade mold part 25 has a complex shape, such that it is impossible for all the required inter-blade mold parts 25 to be assembled into their correct positions by moving each into place in turn either by a radial or an axial movement. In the past, some designs of rotor blade shape have had the consequence that it is impossible to slide the final inter-blade mold part into position at all, because of obstruction of its path by the adjacent mold parts which are already in position. In such cases, it is necessary to create several smaller mold parts, each defining a respective part of the total inter-blade space, and these smaller mold parts each have to be inserted and positioned in turn to fill the final inter-blade space as the mold is assembled. This requires additional patterns, for making the special mold parts, and a more complex assembly operation as the mold parts are assembled into the final inter-blade space. However, if all the rotor blades follow a common screw thread pitch, it must be possible to insert the final inter-blade mold part 25 into its correct position, after all the other inter-blade mold parts 25 have been placed in their positions, by a screw spiral motion so that the blade-defining surfaces of the mold part 25 follow the paths of the blades they define. The screw thread shape of the blades means that the inter-blade mold part 25 will be able to follow this line of movement into its correct position without being obstructed by any other inter-blade mold part 25.

In order to design a radial flow rotor with a concave flared blade-bearing hub surface 3, similar to the rotor of FIGS. 1 to 4, the designer will normally start from the shape of the blade-bearing surface 3 and the hub lines 9. The shape of the blade-bearing surface may be limited to some extent by external considerations, such as the total shape and size of the space allowed for the rotor in the design of a larger installation of which it will form a part. Within such constraints, the designer will use their experience to create a design which is likely to be useful. The designer will particularly need to take into account the velocity and volumetric flow rate of fluid that the rotor is intended to handle and consequently the angle of relative motion between the fluid and the rotor during operation. Normally, the hub lines 9 will be designed to be generally parallel to the intended fluid flow in use at each end of the hub 1, and between the two ends of the hub the hub lines 9 will normally be selected to provide a substantially constant relative speed of the fluid flow, relative to the blade-bearing surface 3 of the hub, or else a smoothly varying speed, in order to avoid localized fluid acceleration or deceleration which can disrupt the smooth flow of fluid through the rotor.

Because the blade-bearing surface 3 of the hub is substantially parallel to the axis of rotation 4 at the narrow end of the hub 1, the angle of the hub lines 9 of the blades at this end of the blade-bearing surface 3 tends to determine the screw pitch dθ/dx. Consequently, the blade shapes are determined by extending the blades away from each point of each hub line maintaining a constant radial distance r from the axis of rotation 4 and following the screw pitch dθ/dx, until the blades reach a predefined virtual surface for the blade tips or shroud lines, or if the shape of the shroud around the rotor is not predefined, until the blades define a shroud shape which the designer deems to be suitable. As mentioned above, it is preferred that the blades are tapered so as to become thinner as they move away from the hub line, in order to facilitate removable of the blades 5, 7 from the mold base part 11. This is achieved by selecting an appropriate draft angle for the blade surfaces as they extend away from the hub line 9. The thickness of the hub lines 9 and the magnitude of the draft angle can be adjusted, if desired, so as to optimize the blade thickness.

The resulting rotor design can then be subjected to fluid dynamic analysis in order to estimate how it will perform in practice, and to assess whether the blade angles of the rotor design are correct. If necessary, the rotor design can be varied by adjusting the hub lines 9, both to change the overall path of the blades 5, 7 over the blade-bearing surface 3 and to adjust the screw pitch dθ/dx. The revised design can then be optimized and subjected to fluid dynamic analysis, and the design adjustment process can be repeated as necessary, until a satisfactory conclusion is reached.

Because the radius of the blade-bearing surface 3 is much greater at the radial flow end of the hub 1 than at the axial flow end of the hub 1, even a slight twist of the hub lines 9 of the blades around the axis of rotation 4 at the axial flow end of the hub will define a screw pitch dθ/dx which causes the radial flow ends of the blades 5, 7 to be swept sideways for a considerable circumferential distance for a small change in axial distance. Consequently, at the circumferentially outermost parts of the rotor, the blades 5, 7 lean over with a considerable rake angle, as is clearly visible in FIG. 2. By comparison, the blades 5, 7 of the known rotor of FIGS. 5 to 8 do not lean over at their radial flow ends where the rotor diameter is largest, as can be seen in FIG. 6. This rake angle or “blade lean” is the inevitable consequence of maintaining a constant screw thread pitch dθ/dx over all parts of all blades 5, 7, together with the need to angle the axial flow ends of the blades 5, 7 to accommodate the direction of relative movement between the blades and the fluid. Since the rake angle can affect the flow of fluid through the rotor, it might be thought that this unusual rake angle would make it difficult to obtain satisfactory fluid dynamic performance of the rotor. However, aerodynamic analysis and subsequent testing of a prototype rotor substantially in accordance with FIGS. 1 to 4 showed that satisfactory performance could be obtained.

The embodiment of FIGS. 1 to 4 has a hub 1 the diameter of which varies with axial distance along the hub, and the rotor is suitable for radial fluid flow. FIGS. 14 to 17 show rotors having cylindrical hubs, which are suitable for axial fluid flow through the rotor. In these rotors, the shape of the hub 1 does not obstruct axial separation of the rotor from a mold part. However, these rotor designs have substantial axial overlap between adjacent rotor blades, thereby preventing separation of the rotor from a two-piece mold by axial movement. Nevertheless, as with the embodiment of FIGS. 1 to 4, the rotors of FIGS. 14 to 17 have rotor blades 7 which conform to a screw thread shape, so that a rotor or molding pattern can be separated from a mold part by a screw motion without destruction of the mold part. Accordingly, the impellers of FIGS. 14 to 17 may also be manufactured by a molding process in which the mold is not destroyed, such as injection molding, with a conveniently small number of mold pieces, for example using a two-piece mold. Additionally, if such impellers are made using a molding or casting process in which the mold is destroyed, such as sand molding or casting into a plaster of Paris mold, the advantages described above for the methods of making or assembling the mold can also be obtained with these designs of rotor.

The examples and embodiments of the present invention that have been described herein are provided by way of non-limiting illustration, and those skilled in the art will understand that many variations are possible that fall within the scope of the present invention as defined by the following claims. 

1. A rotor, for driving a fluid by rotation of the rotor about an axis of rotation or for being driven by a fluid so as to rotate about an axis of rotation, or a pattern for making a mold part for making the rotor, wherein the rotor or pattern comprises (i) a hub having a blade-bearing surface and (ii) a plurality of rotor blades extending from the blade-bearing surface of the hub, the blade bearing surface of the hub having a first surface portion at a first radial distance from said axis of rotation and a second surface portion at a second radial distance, greater than said first distance, from said axis of rotation, the second surface portion being spaced from the axis of rotation in the same angular direction from said axis of rotation as the first surface portion and being spaced from the first surface portion in a direction parallel to the axis of rotation, both the first and second surface portions bearing at least a part of a rotor blade, at least some of the rotor blades having a respective portion that is spaced from a respective portion of another rotor blade in a direction parallel to the axis of rotation but is at the same radial distance from the axis of rotation and is spaced from the axis of rotation in the same angular direction, wherein the value of dθ/dx is constant and the same for the whole of all blades of the rotor or pattern (where x represents distance in a direction parallel to the axis of rotation, and θ represents angular direction perpendicular to the axis of rotation taking the axis of rotation as the origin) except for differences in the slopes of the blade surfaces arising solely from variation in the thickness of the rotor blades.
 2. The rotor or pattern according to claim 1, wherein the blade-bearing surface of the hub has a third surface portion at a third radial distance from the axis of rotation, the third surface portion being spaced from the axis of rotation in the same angular direction from said axis of rotation as the first and second surface portions and being midway between the first and second surface portion in a direction parallel to the axis of rotation, and wherein the third radial distance is less than the average of the first and second radial distances.
 3. The rotor or pattern according to claim 2, wherein the blade-bearing surface of the hub is substantially parallel to the axis of rotation at the first surface portion and the blade-bearing surface of the hub is substantially perpendicular to the axis of rotation at the second surface portion.
 4. The rotor or pattern according to claim 1, wherein said respective portions of at least one pair of rotor blades extend circumferentially with respect to the axis of rotation over an angle of at least 5° subtended at the axis of rotation.
 5. The rotor or pattern according to claim 1, wherein said respective portions of at least one pair of rotor blades extend circumferentially with respect to the axis of rotation over an angle of at least 10° subtended at the axis of rotation.
 6. The rotor pattern according to claim 1, wherein said respective portions of at least one pair of rotor blades extend circumferentially with respect to the axis of rotation over an angle of at least 15° subtended at the axis of rotation.
 7. A rotor, or a pattern for making a mold part for making a rotor, having a hub and a plurality of rotor blades extending from the hub, wherein, for each rotor blade, all portions of the rotor blade at the same radial distance from the axis of rotation of the rotor form part of the same screw thread spiral, and the screw pitch of the screw thread spiral is the same at all radial distances of the blade and is the same for all of the rotor blades, at least some of the rotor blades overlap axially at least partially in the sense that a part of one rotor blade at the same polar co-ordinates from the axis of rotation as a part of another rotor blade but is spaced axially therefrom, and at least a part of the hub has a diameter which varies with distance along the axis of rotation.
 8. Apparatus for driving a gas, the apparatus comprising an impeller that is a rotor according to claim
 1. 9. A turbocharger or supercharger for an internal combustion engine, comprising a rotor according to claim
 1. 10. Apparatus for driving a gas, the apparatus comprising an impeller that is a rotor according to claim
 7. 11. A turbocharger or supercharger for an internal combustion engine, comprising a rotor according to claim
 7. 12. A mold part for use in making a rotor, the mold part having a recess for receiving molding material in use, said recess having recess portions that define both faces of each blade of the rotor, all parts of each said recess portion having a common value of dθ/dx, where x represents distance in a direction parallel to the axis of rotation, and θ represents angular direction perpendicular to the axis of rotation taking the axis of rotation as the origin, except for differences in the slopes of the surfaces of the recess portions arising solely from variation in the width of the recess portions in the direction of the thickness of the rotor blades, and at least some of said recess portions overlapping another said recess portion in the axial direction of the rotor
 13. The mold part according to claim 12, wherein part of the surface of said recess defines a blade-bearing surface of a hub of the rotor, and said part of the surface of said recess has respective portions axially spaced from each other, with reference to the axis of rotation of the rotor, which portions are at different radial distances from said axis of rotation.
 14. The mold part according to claim 13, wherein said part of the recess surface defining a blade-bearing surface is convex so that said part of the recess surface is less parallel to the axis of rotation of the rotor where it is further from said axis of rotation
 15. A method of making a rotor comprising filling the recess of a mold part according to claim 12 with a flowable material, causing or allowing the flowable material to solidify, and removing the solidified material from the mold part by moving it relative to the mold part with a screw motion.
 16. The method according to claim 15, wherein the step of filling a mold part with a flowable material comprises injecting the flowable material under pressure.
 17. The method according to claim 15, wherein said flowable material is a settable plastics resin.
 18. The method according to claim 17, wherein the plastics resin contains re-inforcing material.
 19. A method of making a mold part according to claim 12 comprising immersing or embedding a rotor pattern in a mold-forming material, allowing or causing the mold-forming material to solidify, and withdrawing the pattern from the solidified material by movement with a relative screw motion to leave the recess in the solidified material.
 20. The method according to claim 19, wherein the mold-forming material is or mostly comprises sand.
 21. A method of making a rotor comprising making a mold part by a method according to claim 19, filling the recess left by the pattern with a molding material, allowing or causing the molding material to solidify, and releasing the solidified molding material by destroying the mold part.
 22. The method according to claim 21, wherein the molding material is molten metal, and the step of allowing or causing the molding material to solidify comprises allowing or causing the molten metal to cool.
 23. An inter-blade mold part for a mold for use in making a rotor by molding or casting, the inter-blade mold part having a first surface that defines a surface on one side of a blade of a rotor and a second surface that defines a surface on the other side of another blade of the rotor, wherein for both of the first and second surfaces of the inter-blade part, all parts of the surface that define a blade surface of a rotor slope at a common screw pitch, with reference to an axis of rotation of the rotor, except for differences in the slopes of the first and second surfaces arising solely from variation in the thickness of said blades.
 24. The inter-blade mold part according to claim 23 which is or mostly comprises sand.
 25. A sand mold forming pattern for defining an inter-blade sand mold part according to claim
 24. 